1. Field of the Invention
This invention relates to a rotor for a reluctance type electric rotating machine which can achieve the similar effects to those achieved by skew.
2. Description of the Related Art
A reluctance type rotating machine or, for example, a reluctance type rotating machine provided with permanent magnets includes a rotor formed with a magnetic convex portion where a flux is easy to pass (d axis) and a magnetic concave portion where a flux is difficult to pass (q axis) and a permanent magnet which is disposed in a stator provided with a stator winding. The magnetic convex portion (d axis) has a high magnetic flux density in an air gap, whereas the magnetic concave portion (q axis) has a low magnetic flux density in an air gap. These variations in the magnetic flux density produce reluctance torque. Furthermore, torque is also developed by a magnetic attractive force and a magnetic repulsive force between poles of the permanent magnet and stator.
FIGS. 7 and 8 illustrate an example of conventional rotor for a reluctance type rotating machine with permanent magnets. The illustrated machine is an 8-pole machine. FIG. 7 is a side view of the rotor with an end plate and a rotational shaft being eliminated. FIG. 8 is a sectional view taken along line 8—8 in FIG. 7. Referring to FIG. 7, the rotor 100 includes a rotor core 101 made by stacking a number of annular silicon steel sheets. The rotor core 101 has pairs of generally rectangular magnet insertion holes 102 formed in an outer circumference thereof as shown in FIG. 8. Permanent magnets 103 are inserted and fixed in the insertion holes 102 respectively. The outer circumference of the rotor core 101 is further formed with cavities 104 located between the respective pairs of permanent magnets 103 as shown in FIG. 8. Each cavity 104 is formed into a generally triangular shape. In the rotor 100, each pair of insertion holes 102, permanent magnets 103 and each cavity 104 constitute the aforesaid magnetic concave portion 105 where a flux is difficult to pass (q axis). Each portion between the concave portions 105 constitutes the aforesaid magnetic convex portion 106 where a flux is easy to pass (d axis). The magnetic concave and convex portions 105 and 106 are formed alternately with a predetermined angle therebetween. See JP-A-2000-339922, for example.
The rotor core 101 has keys 107 formed on an inner circumference thereof. The keys 107 are adapted to engage key grooves of a rotational shaft respectively. Furthermore, a center line Lo passing the keys 107 is adapted to pass the center of the magnetic convex portion 106. A center line La passes the center of the magnetic convex portion 105 adjacent to the center line Lo. The center line Lo is adapted to meet the center line La at an angle θ. The angle θ is at 22.5 degrees when the rotor 100 has 8 poles. The rotor 100 is adapted to be disposed in a stator (not shown) provided with a stator winding.
It is well known that squirrel-cage induction motors result in crawling due to torque developed by high harmonics. There is a possibility that permanent-magnet reluctance type rotating machines as the reluctance type rotating machine may cause the similar crawling to that caused by the squirrel-cage induction motors. As a result, the crawling results in torque ripple, oscillation, vibration and noise.